Go to koepi's site here
http://www.koepi.org/
and download the file XviD-1.0.3-20122004.exe from there. Save it anywhere on your disk. It doesn't matter where, but don't forget where you save it. I use a folder named KITS. After that, double click on it to install XviD update. When asked about decoding all supported formats, uncheck the box. That's it.